1. Field of the Invention
The invention relates to a process for the production of catalysts for the reduction of nitrogen oxides from exhaust gases and similar chemical air cleaning processes, in which a catalytically active material is deposited and bonded on a carrier made of metal, preferably stainless steel, in the form of a perforated metal plate, of wire mesh, woven, honeycombed or knitted material made of metal wire, or a similar material with a high specific surface area.
2. Description of the Prior Art
German Patent Publication Published for Opposition Purposes No. DE-AS 23 53 640, which is equivalent to U.S. Pat. No. 3,956,192, entitled "Catalyst Carrier", describes a catalyst carrier with a blank comprising metal wire distributed as uniformly as possible over the volume between support mats, whereby the blank exhibits several layers packed on top of one another and attached to one another consisting of a woven, honeycombed or knitted endless alloy wire, heat resistant up to 800.degree. C.
German Patent Publication Published for Opposition Purposes No. DE-AS 28 46 476 describes a process for the manufacture of sintered blanks which contain titanium dioxide, an active component, and molybdenum oxide as binders for the titanium dioxide, whereby the vaporous molybdenum oxide is deposited on pulverized titanium oxide in an atmosphere of molybdenum oxide vapor at temperatures from 460.degree. C. to 650.degree. C. In this manner, a pulverized mixture is produced which contains the pulverized titanium oxide with the molybdenum oxide deposited on it and the active components, and a quantity of water is added to it so that a viscous mixture is formed. A blank is produced from the kneaded mixture and the blank is calcined to form a sintered product. In this proposal, for example, a plate structure is formed by applying a viscous material, obtained by adding 25 to 35 percent by weight, preferably 24 to 28 percent by weight, water to the pulverized mixture on a porous steel plate. Other plate structures can also be formed, by applying viscous material obtained by adding 20 to 30 percent by weight water to various metal wire meshes.
German Patent No. DE-PS 28 53 023 describes a plate-shaped catalyst, comprising a carrier, a metal intermediate layer applied to it, and a catalytically active component of the prior art deposited on it suitable for the reduction of oxides of nitrogen, NO.sub.x, in an exhaust gas, whereby both surfaces of the plate are roughened by spraying these on molten metal. Catalytic substances which can be used for the reduction of the oxides of nitrogen, NO.sub.x, contained in an exhaust gas by means of ammonia include titanium and vanadium, or titanium, tungsten and one or more of the elements vanadium, molybdenum or iron. Such catalytic substances were processed prior to the proposal of DE-PS 28 53 023, for example, as hydroxide or water-soluble salt into a suspension or paste, which was combined as necessary with a suitable carrier material, to apply the suspension or paste obtained in the form of a coating to a substrate for the catalyst, and then drying or sintering it, to bind it for use to the substrate. Catalytic substances were also bonded to the substrate in the form of oxides or sulfates. DE-PS 28 53 023 was intended to demonstrate that portions of the catalytically active substances deposited on the opposite sides of the carrier plate are in connection with one another through the perforation, to make certain that the catalytically active substance deposited on the carrier plate adheres firmly to it.
German Patent No. DE-PS 29 27 246 describes a plate-shaped catalyst for the reduction of nitrogen oxides in exhaust gases with a porous, firmly-adhering metal oxide carrier deposited on metal. A catalytically active component is then applied to the carrier, whereby the catalyst is manufactured by the application of a suspension containing particulate titanium oxide and colloidal silicic acid in a weight ratio of 1:10 to 10:1 on a metal mesh with an open mesh width of 0.147 millimeters to 1.651 millimeters, and whereby the active component is incorporated before, during or after the application. The suspension is thereby formed out of finely-divided titanium oxide and the binder containing the colloidal silicic acid, and is deposited on the metal mesh. The resulting carrier is then dried and immersed in a solution of a salt of the active component, and then dried and fired, if necessary. It is also possible to immerse the metal grid in a coating bath, which contains titanium oxide and binder, the metal grid is then dried, and the carrier formed is immersed in a solution of a salt of the active component. It can then be dried and fired if necessary. It is also possible to immerse the metal grid in a coating bath which contains titanium oxide and binder, then to dry the metal grid and immerse the carrier thus formed in a solution of a salt of the active component, and then to dry and fire it if necessary.
All the above-cited documents are incorporated herein by reference as if set forth herein in their entirety.
The processes of the prior art for the manufacture of catalysts thus entail a comparatively high manufacturing expense to produce catalysts which meet the requirements of large open flow cross sections and large surface area relative to volume, although the problem of the adherence of the catalytically active material is still not solved in a satisfactory manner.